The present invention relates to solar energy collectors and more particularly to concentrating solar energy collectors which concentrate incident sunlight onto an element.
Solar energy collectors generally transform incident solar radiation to another energy form, typically heat or electricity. Two principal types of collectors are concentrating collectors and flat panel collectors. Concentrating collectors typically have a curved reflector or lens which is positioned to focus incident solar radiation onto a collection element such as a heat absorption pipe or photovoltaic cell. Flat panel collectors, as the name suggests, are flat and generally do not concentrate the incident energy onto the collection element. In systems where solar radiation is converted into heat energy, hereinafter referred to as "thermal collection" systems, the heat energy is often used to heat a working fluid such as water. The working fluid is typically carried within a heat absorption pipe or is otherwise carried in close physical proximity to some other heat collection element.
Concentrating solar energy collectors can have several significant advantages over flat panel collectors, particularly in high temperature thermal collection applications. One such advantage is that it is generally possible to raise the working fluid in a concentrating collector to a higher temperature than that which is typically obtained by a flat collector. This is attributable to the ability of concentrating collectors to provide a greater amount of energy per unit area of the heat collection element than that typically obtained with unconcentrated sunlight. Second, the heat absorption pipe can be significantly more expensive per unit area than the cost of the reflecting material which is used to concentrate the solar radiation onto the heat absorption pipe. Hence, fewer pipes may be used with a resultant cost saving. Furthermore, convection and conduction heat losses are related to the total surface area of the heat collection element and therefore can be significantly greater in non-concentrating systems where the size or the number of the collection elements is typically much greater.
Concentrating solar collectors have a range of "concentration ratios" which compare the total collecting area of the collector to the collection element cross-sectional area on which the radiation is focused. These ratios are often measured in units of "suns". Concentrating collectors with concentration ratios of four suns to one or less are often referred to as low concentration collectors. On the other hand, high concentration ratio collectors can range as high as 20 suns to one, to 60 suns to one, or even higher.
A disadvantage experienced with high concentration collectors has been the failure of the high concentration collectors to utilize the diffused component of the incident solar radiation. Solar radiation is generally comprised of two components, direct radiation, which generally travels in parallel paths, and diffused radiation which is scattered. The direct radiation component can be focused accurately by various types of lenses or reflectors. The diffused radiation component, which can constitute from 12% to 25% of the total incident radiation depending upon atmospheric conditions, typically cannot be accurately focused. Because flat collectors are inherently able to collect diffused radiation, many research efforts have concerned themselves exclusively with low concentration or flat panel collectors, feeling there was a fundamental 12 to 25% energy collection advantage over high energy concentration collectors.
Concentrating collection systems are usually mounted onto sun tracking mechanisms which maintain the collectors pointed at the sun as the sun appears to move across the sky. The generally higher temperatures possible with a tracking thermal collection system can justify the additional expense. This has not generally been the case with photovoltaic systems or other systems which convert solar radiation directly to electricity. The additional electricity produced generally has not economically justified mounting these systems on a sun tracking structure.